The present invention relates to vacuum circuit interrupters used for electrical power distribution. Such devices are used to carry high currents during normal closed switch contact operation, and to interrupt this current flow by opening the contacts in a vacuum. These vacuum circuit interrupters are designed for reliable, reusable operations in for example 40 kiloamps at 38 kilovolt systems. The current carrying contacts must be designed to have the capacity of handling such large currents, yet be able to interrupt this current when the contacts are separated without restriking of the arc with such high voltages across the contacts.
The formation of an anode spot and the anode contact melting associated with it constitutes the fundamental plysical mechanism which limits the high current interruption capability for vacuum circuit interrupters. The simple planar butt type contact, even with very large diameter contacts, has been found ineffective to interrupt such high currents and voltages.
A variety of contact structures have been tried in the prior art to move or rotate the formed arc to avoid anode spot formation, or at least to move the spot over the anode surface to minimize anode melting. These structures range from forming the contacts with spiral arms to forming cup-shaped contacts with slotted arc surface rims as in U.S. Pat. No. 4,149,050. The use of an axial magnetic field parallel to the arc path has been found to maintain the arc in a more diffuse mode than would otherwise be the case for the same current levels without the axial magnetic field.
The use of an auxiliary arcing electrode about the two primary electrodes to provide a plural arc gap path is taught by U.S. Pat. No. 3,612,795.
It is desirable to provide a vacuum circuit interrupter with a contact structure that carries high operating current with minimum losses and can reliably interrupt high current at high voltage without restriking or eroding the contacts.